flex is a tool for generating
scanners: programs which recognize lexical
patterns in text. flex reads the given
input files, or its standard input if no file names are given, for a
description of a scanner to generate. The description is in the form of pairs
of regular expressions and C code, called rules.
flex generates as output a C source file,
lex.yy.c, which defines a routine
yylex(). This file is compiled and linked
with the -lfl library to produce an
executable. When the executable is run, it analyzes its input for occurrences
of the regular expressions. Whenever it finds one, it executes the
corresponding C code.

lex is a synonym for
flex.
flex++ is a synonym for
flex-+.

The manual includes both tutorial and reference sections:

Some Simple Examples

Format of the Input File

Patterns

The extended regular expressions used by
flex.

How the Input is Matched

The rules for determining what has been matched.

Actions

How to specify what to do when a pattern is matched.

The Generated Scanner

Details regarding the scanner that flex
produces; how to control the input source.

Start Conditions

Introducing context into scanners, and managing
“mini-scanners”.

Multiple Input Buffers

How to manipulate multiple input sources; how to scan from strings instead
of files.

First some simple examples to get the flavor of how one uses
flex. The following
flex input specifies a scanner which
whenever it encounters the string “username” will replace it
with the user's login name:

%%
username printf("%s", getlogin());

By default, any text not matched by a flex
scanner is copied to the output, so the net effect of this scanner is to copy
its input file to its output with each occurrence of “username”
expanded. In this input, there is just one rule. “username” is
the pattern and the “printf” is the
action. The “%%” marks the
beginning of the rules.

This scanner counts the number of characters and the number of lines in its
input (it produces no output other than the final report on the counts). The
first line declares two globals, “num_lines” and
“num_chars”, which are accessible both inside
yylex() and in the
main() routine declared after the second
“%%”. There are two rules, one which matches a newline
("\n") and increments both the line count and the character count,
and one which matches any character other than a newline (indicated by the
“.” regular expression).

The flex input file consists of three
sections, separated by a line with just “%%” in it:

definitions
%%
rules
%%
user code

The definitions section contains declarations of
simple name definitions to simplify the scanner
specification, and declarations of start
conditions, which are explained in a later section.

Name definitions have the form:

name definition

The “name” is a word beginning with a letter or an underscore
(‘_’) followed by zero or more letters, digits,
‘_’, or ‘-’ (dash). The definition is taken to
begin at the first non-whitespace character following the name and continuing
to the end of the line. The definition can subsequently be referred to using
“{name}”, which will expand to “(definition)”. For
example:

DIGIT [0-9]
ID [a-z][a-z0-9]*

This defines “DIGIT” to be a regular expression which matches a
single digit, and “ID” to be a regular expression which matches
a letter followed by zero-or-more letters-or-digits. A subsequent reference to

{DIGIT}+"."{DIGIT}*

is identical to

([0-9])+"."([0-9])*

and matches one-or-more digits followed by a ‘.’ followed by
zero-or-more digits.

The rules section of the
flex input contains a series of rules of
the form:

pattern action

The pattern must be unindented and the action must begin on the same line.

See below for a further description of patterns and actions.

Finally, the user code section is simply copied to
lex.yy.c verbatim. It is used for companion
routines which call or are called by the scanner. The presence of this section
is optional; if it is missing, the second “%%” in the input file
may be skipped too.

In the definitions and rules sections, any indented text or text enclosed in
‘%{’ and ‘%}’ is copied verbatim to the output
(with the %{}'s removed). The %{}'s must appear unindented on lines by
themselves.

In the rules section, any indented or %{} text appearing before the first rule
may be used to declare variables which are local to the scanning routine and
(after the declarations) code which is to be executed whenever the scanning
routine is entered. Other indented or %{} text in the rule section is still
copied to the output, but its meaning is not well-defined and it may well
cause compile-time errors (this feature is present for POSIX compliance; see
below for other such features).

In the definitions section (but not in the rules section), an unindented comment
(i.e., a line beginning with “/*”) is also copied verbatim to
the output up to the next “*/”.

The patterns in the input are written using an extended set of regular
expressions. These are:

x

Match the character ‘x’.

.

Any character (byte) except newline.

[xyz]

A “character class”; in this case, the pattern matches
either an ‘x’, a ‘y’, or a
‘z’.

[abj-oZ]

A “character class” with a range in it; matches an
‘a’, a ‘b’, any letter from ‘j’
through ‘o’, or a ‘Z’.

[^A-Z]

A “negated character class”, i.e., any character but those
in the class. In this case, any character EXCEPT an uppercase letter.

[^A-Z\n]

Any character EXCEPT an uppercase letter or a newline.

r*

Zero or more r's, where ‘r’ is any regular expression.

r+

One or more r's.

r?

Zero or one r's (that is, “an optional r”).

r{2,5}

Anywhere from two to five r's.

r{2,}

Two or more r's.

r{4}

Exactly 4 r's.

{name}

The expansion of the “name” definition (see above).

"[xyz]\"foo"

The literal string: [xyz]"foo.

\X

If ‘X’ is an ‘a’, ‘b’,
‘f’, ‘n’, ‘r’,
‘t’, or ‘v’, then the ANSI-C interpretation of
‘\X’. Otherwise, a literal ‘X’ (used to escape
operators such as ‘*’).

\0

A NUL character (ASCII code 0).

\123

The character with octal value 123.

\x2a

The character with hexadecimal value 2a.

(r)

Match an ‘r’; parentheses are used to override precedence
(see below).

rs

The regular expression ‘r’ followed by the regular
expression ‘s’; called “concatenation”.

r|s

Either an ‘r’ or an ‘s’.

r/s

An ‘r’, but only if it is followed by an ‘s’.
The text matched by ‘s’ is included when determining whether
this rule is the “longest match”, but is then returned to
the input before the action is executed. So the action only sees the text
matched by ‘r’. This type of pattern is called
“trailing context”. (There are some combinations of r/s that
flex cannot match correctly; see notes
in the BUGS section below
regarding “dangerous trailing context”.)

^r

An ‘r’, but only at the beginning of a line (i.e., just
starting to scan, or right after a newline has been scanned).

r$

An ‘r’, but only at the end of a line (i.e., just before a
newline). Equivalent to “r/\n”.

Note that flex's notion of
“newline” is exactly whatever the C compiler used to compile
flex interprets ‘\n’
as.

<s>r

An ‘r’, but only in start condition ‘s’ (see
below for discussion of start conditions).

<s1,s2,s3>r

The same, but in any of start conditions s1, s2, or s3.

<*>r

An ‘r’ in any start condition, even an exclusive one.

<<EOF>>

An end-of-file.

<s1,s2><<EOF>>

An end-of-file when in start condition s1 or s2.

Note that inside of a character class, all regular expression operators lose
their special meaning except escape (‘\’) and the character
class operators, ‘-’, ‘]’, and, at the beginning
of the class, ‘^’.

The regular expressions listed above are grouped according to precedence, from
highest precedence at the top to lowest at the bottom. Those grouped together
have equal precedence. For example,

foo|bar*

is the same as

(foo)|(ba(r*))

since the ‘*’ operator has higher precedence than concatenation,
and concatenation higher than alternation (‘|’). This pattern
therefore matches either the string
“foo” or the string
“ba” followed by zero-or-more r's. To match “foo”
or zero-or-more "bar"'s, use:

foo|(bar)*

and to match zero-or-more "foo"'s-or-"bar"'s:

(foo|bar)*

In addition to characters and ranges of characters, character classes can also
contain character class expressions. These are
expressions enclosed inside ‘[:’ and ‘:]’
delimiters (which themselves must appear between the ‘[’ and
‘]’ of the character class; other elements may occur inside the
character class, too). The valid expressions are:

These expressions all designate a set of characters equivalent to the
corresponding standard C isXXX() function.
For example, [:alnum:] designates those characters for which
isalnum(3) returns true - i.e.,
any alphabetic or numeric. Some systems don't provide
isblank(3), so
flex defines [:blank:] as a blank or a tab.

For example, the following character classes are all equivalent:

[[:alnum:]]
[[:alpha:][:digit:]]
[[:alpha:]0-9]
[a-zA-Z0-9]

If the scanner is case-insensitive (the -i
flag), then [:upper:] and [:lower:] are equivalent to [:alpha:].

Some notes on patterns:

A negated character class such as the example “[^A-Z]” above
will match a newline unless "\n" (or an equivalent escape
sequence) is one of the characters explicitly present in the negated
character class (e.g., “[^A-Z\n]”). This is unlike how many
other regular expression tools treat negated character classes, but
unfortunately the inconsistency is historically entrenched. Matching
newlines means that a pattern like “[^"]*” can match
the entire input unless there's another quote in the input.

A rule can have at most one instance of trailing context (the
‘/’ operator or the ‘$’ operator). The start
condition, ‘^’, and “<<EOF>>”
patterns can only occur at the beginning of a pattern and, as well as with
‘/’ and ‘$’, cannot be grouped inside
parentheses. A ‘^’ which does not occur at the beginning of
a rule or a ‘$’ which does not occur at the end of a rule
loses its special properties and is treated as a normal character.

The following are illegal:

foo/bar$
<sc1>foo<sc2>bar

Note that the first of these, can be written “foo/bar\n”.

The following will result in ‘$’ or ‘^’ being
treated as a normal character:

foo|(bar$)
foo|^bar

If what's wanted is a “foo” or a bar-followed-by-a-newline,
the following could be used (the special ‘|’ action is
explained below):

foo |
bar$ /* action goes here */

A similar trick will work for matching a foo or a
bar-at-the-beginning-of-a-line.

When the generated scanner is run, it analyzes its input looking for strings
which match any of its patterns. If it finds more than one match, it takes the
one matching the most text (for trailing context rules, this includes the
length of the trailing part, even though it will then be returned to the
input). If it finds two or more matches of the same length, the rule listed
first in the flex input file is chosen.

Once the match is determined, the text corresponding to the match (called the
token) is made available in the global character
pointer yytext, and its length in the global
integer yyleng. The
action corresponding to the matched pattern is
then executed (a more detailed description of actions follows), and then the
remaining input is scanned for another match.

If no match is found, then the default rule is executed: the next character in
the input is considered matched and copied to the standard output. Thus, the
simplest legal flex input is:

%%

which generates a scanner that simply copies its input (one character at a time)
to its output.

Note that yytext can be defined in two
different ways: either as a character pointer or as a character array. Which
definition flex uses can be controlled by
including one of the special directives “%pointer” or
“%array” in the first (definitions) section of flex input. The
default is “%pointer”, unless the
-llex
compatibility option is used, in which case
yytext will be an array. The advantage of
using “%pointer” is substantially faster scanning and no buffer
overflow when matching very large tokens (unless not enough dynamic memory is
available). The disadvantage is that actions are restricted in how they can
modify yytext (see the next section), and
calls to the unput() function destroy the
present contents of yytext, which can be a
considerable porting headache when moving between different
lex versions.

The advantage of “%array” is that
yytext can be modified as much as wanted, and
calls to unput() do not destroy
yytext (see below). Furthermore, existing
lex programs sometimes access
yytext externally using declarations of the
form:

extern char yytext[];

This definition is erroneous when used with “%pointer”, but
correct for “%array”.

“%array” defines yytext to be an
array of YYLMAX characters, which defaults
to a fairly large value. The size can be changed by simply #define'ing
YYLMAX to a different value in the first
section of flex input. As mentioned above,
with “%pointer” yytext grows dynamically to accommodate large
tokens. While this means a “%pointer” scanner can accommodate
very large tokens (such as matching entire blocks of comments), bear in mind
that each time the scanner must resize yytext
it also must rescan the entire token from the beginning, so matching such
tokens can prove slow. yytext presently does
not dynamically grow if a call to unput()
results in too much text being pushed back; instead, a run-time error results.

Also note that “%array” cannot be used with C++ scanner classes
(the c++ option; see below).

Each pattern in a rule has a corresponding action, which can be any arbitrary C
statement. The pattern ends at the first non-escaped whitespace character; the
remainder of the line is its action. If the action is empty, then when the
pattern is matched the input token is simply discarded. For example, here is
the specification for a program which deletes all occurrences of “zap
me” from its input:

%%
"zap me"

(It will copy all other characters in the input to the output since they will be
matched by the default rule.)

Here is a program which compresses multiple blanks and tabs down to a single
blank, and throws away whitespace found at the end of a line:

%%
[ \t]+ putchar(' ');
[ \t]+$ /* ignore this token */

If the action contains a ‘{’, then the action spans till the
balancing ‘}’ is found, and the action may cross multiple lines.
flex knows about C strings and comments and
won't be fooled by braces found within them, but also allows actions to begin
with ‘%{’ and will consider the action to be all the text up to
the next ‘%}’ (regardless of ordinary braces inside the action).

An action consisting solely of a vertical bar (‘|’) means
“same as the action for the next rule”. See below for an
illustration.

Actions can include arbitrary C code, including return statements to return a
value to whatever routine called yylex().
Each time yylex() is called, it continues
processing tokens from where it last left off until it either reaches the end
of the file or executes a return.

Actions are free to modify yytext except for
lengthening it (adding characters to its end - these will overwrite later
characters in the input stream). This, however, does not apply when using
“%array” (see above); in that case,
yytext may be freely modified in any way.

Actions are free to modify yyleng except they
should not do so if the action also includes use of
yymore() (see below).

There are a number of special directives which can be included within an action:

ECHO

Copies yytext to the scanner's
output.

BEGIN

Followed by the name of a start condition, places the scanner in the
corresponding start condition (see below).

REJECT

Directs the scanner to proceed on to the “second best” rule
which matched the input (or a prefix of the input). The rule is chosen as
described above in
HOW THE INPUT IS
MATCHED, and yytext and
yyleng set up appropriately. It may
either be one which matched as much text as the originally chosen rule but
came later in the flex input file, or
one which matched less text. For example, the following will both count
the words in the input and call the routine
special() whenever “frob”
is seen:

Without the REJECT, any "frob"'s in
the input would not be counted as words, since the scanner normally
executes only one action per token. Multiple
REJECT's are allowed, each one finding the
next best choice to the currently active rule. For example, when the
following scanner scans the token “abcd”, it will write
“abcdabcaba” to the output:

(The first three rules share the fourth's action since they use the special
‘|’ action.) REJECT is a
particularly expensive feature in terms of scanner performance; if it is
used in any of the scanner's actions it will slow down all of the
scanner's matching. Furthermore, REJECT
cannot be used with the -Cf or
-CF options (see below).

Note also that unlike the other special actions,
REJECT is a
branch; code immediately following it in the
action will not be executed.

yymore()

Tells the scanner that the next time it matches a rule, the corresponding
token should be appended onto the current value of
yytext rather than replacing it. For
example, given the input “mega-kludge” the following will
write “mega-mega-kludge” to the output:

%%
mega- ECHO; yymore();
kludge ECHO;

First “mega-” is matched and echoed to the output. Then
“kludge” is matched, but the previous “mega-”
is still hanging around at the beginning of
yytext so the
ECHO for the “kludge” rule will
actually write “mega-kludge”.

Two notes regarding use of yymore():
First, yymore() depends on the value of
yyleng correctly reflecting the size of
the current token, so yyleng must not be
modified when using yymore(). Second,
the presence of yymore() in the
scanner's action entails a minor performance penalty in the scanner's
matching speed.

yyless(n)

Returns all but the first n characters of
the current token back to the input stream, where they will be rescanned
when the scanner looks for the next match.
yytext and
yyleng are adjusted appropriately (e.g.,
yyleng will now be equal to
n). For example, on the input
“foobar” the following will write out
“foobarbar”:

%%
foobar ECHO; yyless(3);
[a-z]+ ECHO;

An argument of 0 to yyless will cause the
entire current input string to be scanned again. Unless how the scanner
will subsequently process its input has been changed (using
BEGIN, for example), this will result in an
endless loop.

Note that yyless is a macro and can only be
used in the flex input file, not from
other source files.

unput(c)

Puts the character c back into the input
stream. It will be the next character scanned. The following action will
take the current token and cause it to be rescanned enclosed in
parentheses.

Note that since each unput() puts the
given character back at the beginning of the input stream, pushing back
strings must be done back-to-front.

An important potential problem when using
unput() is that if using
“%pointer” (the default), a call to
unput() destroys the contents of
yytext, starting with its rightmost
character and devouring one character to the left with each call. If the
value of yytext should be preserved after
a call to unput() (as in the above
example), it must either first be copied elsewhere, or the scanner must be
built using “%array” instead (see
HOW THE INPUT IS
MATCHED).

Finally, note that EOF cannot be put back to attempt to mark the input
stream with an end-of-file.

input()

Reads the next character from the input stream. For example, the following
is one way to eat up C comments:

(Note that if the scanner is compiled using C++, then
input() is instead referred to as
yyinput(), in order to avoid a name
clash with the C++ stream by the name of input.)

YY_FLUSH_BUFFER

Flushes the scanner's internal buffer so that the next time the scanner
attempts to match a token, it will first refill the buffer using
YY_INPUT (see
THE GENERATED
SCANNER, below). This action is a special case of the more general
yy_flush_buffer() function, described
below in the section
MULTIPLE INPUT
BUFFERS.

yyterminate()

Can be used in lieu of a return statement in an action. It terminates the
scanner and returns a 0 to the scanner's caller, indicating “all
done”. By default, yyterminate()
is also called when an end-of-file is encountered. It is a macro and may
be redefined.

The output of flex is the file
lex.yy.c, which contains the scanning
routine yylex(), a number of tables used by
it for matching tokens, and a number of auxiliary routines and macros. By
default, yylex() is declared as follows:

int yylex()
{
... various definitions and the actions in here ...
}

(If the environment supports function prototypes, then it will be "int
yylex(void)".) This definition may be changed by defining the
YY_DECL macro. For example:

#define YY_DECL float lexscan(a, b) float a, b;

would give the scanning routine the name lexscan,
returning a float, and taking two floats as arguments. Note that if arguments
are given to the scanning routine using a K&R-style/non-prototyped
function declaration, the definition must be terminated with a semi-colon
(‘;’).

Whenever yylex() is called, it scans tokens
from the global input file yyin (which
defaults to stdin). It continues until it either reaches an end-of-file (at
which point it returns the value 0) or one of its actions executes a
return statement.

If the scanner reaches an end-of-file, subsequent calls are undefined unless
either yyin is pointed at a new input file (in
which case scanning continues from that file), or
yyrestart() is called.
yyrestart() takes one argument, a
FILE * pointer (which can be nil, if
YY_INPUT has been set up to scan from a
source other than yyin), and initializes
yyin for scanning from that file. Essentially
there is no difference between just assigning
yyin to a new input file or using
yyrestart() to do so; the latter is
available for compatibility with previous versions of
flex, and because it can be used to switch
input files in the middle of scanning. It can also be used to throw away the
current input buffer, by calling it with an argument of
yyin; but better is to use
YY_FLUSH_BUFFER (see above). Note that
yyrestart() does not reset the start
condition to INITIAL (see
START CONDITIONS,
below).

If yylex() stops scanning due to executing a
return statement in one of the actions, the
scanner may then be called again and it will resume scanning where it left
off.

By default (and for purposes of efficiency), the scanner uses block-reads rather
than simple getc(3) calls to read
characters from yyin. The nature of how it gets
its input can be controlled by defining the
YY_INPUT macro.
YY_INPUT's calling sequence is
“YY_INPUT(buf,result,max_size)”. Its action is to place up to
max_size characters in the character array
buf and return in the integer variable
result either the number of characters read or
the constant YY_NULL (0 on
UNIX systems) to indicate
EOF. The default
YY_INPUT reads from the global file-pointer
“yyin”.

A sample definition of YY_INPUT (in the
definitions section of the input file):

This definition will change the input processing to occur one character at a
time.

When the scanner receives an end-of-file indication from
YY_INPUT, it then checks the
yywrap() function. If
yywrap() returns false (zero), then it is
assumed that the function has gone ahead and set up
yyin to point to another input file, and scanning
continues. If it returns true (non-zero), then the scanner terminates,
returning 0 to its caller. Note that in either case, the start condition
remains unchanged; it does not revert to INITIAL.

If you do not supply your own version of
yywrap(), then you must either use
“%option noyywrap” (in which case the scanner behaves as though
yywrap() returned 1), or you must link with
-lfl to obtain the default version of the
routine, which always returns 1.

Three routines are available for scanning from in-memory buffers rather than
files: yy_scan_string(),
yy_scan_bytes(), and
yy_scan_buffer(). See the discussion of
them below in the section
MULTIPLE INPUT
BUFFERS.

The scanner writes its ECHO output to the
yyout global (default, stdout), which may be
redefined by the user simply by assigning it to some other
FILE pointer.

flex provides a mechanism for conditionally
activating rules. Any rule whose pattern is prefixed with
“⟨sc⟩” will only be active when the scanner is in
the start condition named “sc”. For example,

<STRING>[^"]* { /* eat up the string body ... */
...
}

will be active only when the scanner is in the “STRING” start
condition, and

<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
...
}

will be active only when the current start condition is either
“INITIAL”, “STRING”, or “QUOTE”.

Start conditions are declared in the definitions (first) section of the input
using unindented lines beginning with either ‘%s’ or
‘%x’ followed by a list of names. The former declares
inclusive start conditions, the latter
exclusive start conditions. A start condition is
activated using the BEGIN action. Until the next
BEGIN action is executed, rules with the given
start condition will be active and rules with other start conditions will be
inactive. If the start condition is inclusive, then rules with no start
conditions at all will also be active. If it is exclusive, then only rules
qualified with the start condition will be active. A set of rules contingent
on the same exclusive start condition describe a scanner which is independent
of any of the other rules in the flex
input. Because of this, exclusive start conditions make it easy to specify
“mini-scanners” which scan portions of the input that are
syntactically different from the rest (e.g., comments).

If the distinction between inclusive and exclusive start conditions is still a
little vague, here's a simple example illustrating the connection between the
two. The set of rules:

Without the ⟨INITIAL,example⟩ qualifier, the “bar”
pattern in the second example wouldn't be active (i.e., couldn't match) when
in start condition “example”. If we just used
⟨example⟩ to qualify “bar”, though, then it would
only be active in “example” and not in
INITIAL, while in the first example it's active
in both, because in the first example the “example” start
condition is an inclusive (‘%s’) start condition.

Also note that the special start-condition specifier
‘⟨*⟩’ matches every start condition. Thus, the
above example could also have been written:

“BEGIN(0)” returns to the original state where only the rules with
no start conditions are active. This state can also be referred to as the
start-condition INITIAL, so
“BEGIN(INITIAL)” is equivalent to “BEGIN(0)”. (The
parentheses around the start condition name are not required but are
considered good style.)

BEGIN actions can also be given as indented code at
the beginning of the rules section. For example, the following will cause the
scanner to enter the “SPECIAL” start condition whenever
yylex() is called and the global variable
enter_special is true:

To illustrate the uses of start conditions, here is a scanner which provides two
different interpretations of a string like “123.456”. By default
it will treat it as three tokens: the integer “123”, a dot
(‘.’), and the integer “456”. But if the string is
preceded earlier in the line by the string “expect-floats” it
will treat it as a single token, the floating-point number 123.456:

This scanner goes to a bit of trouble to match as much text as possible with
each rule. In general, when attempting to write a high-speed scanner try to
match as much as possible in each rule, as it's a big win.

Note that start-condition names are really integer values and can be stored as
such. Thus, the above could be extended in the following fashion:

Often, such as in some of the examples above, a whole bunch of rules are all
preceded by the same start condition(s).
flex makes this a little easier and cleaner
by introducing a notion of start condition scope.
A start condition scope is begun with:

<SCs>{

where “SCs” is a list of one or more start conditions. Inside the
start condition scope, every rule automatically has the prefix
⟨SCs⟩ applied to it, until a ‘}’ which matches the
initial ‘{’. So, for example,

Some scanners (such as those which support “include” files)
require reading from several input streams. As
flex scanners do a large amount of
buffering, one cannot control where the next input will be read from by simply
writing a YY_INPUT which is sensitive to
the scanning context. YY_INPUT is only
called when the scanner reaches the end of its buffer, which may be a long
time after scanning a statement such as an “include” which
requires switching the input source.

To negotiate these sorts of problems, flex
provides a mechanism for creating and switching between multiple input
buffers. An input buffer is created by using:

YY_BUFFER_STATE yy_create_buffer(FILE *file, int
size)

which takes a FILE pointer and a
size and creates a buffer associated with the
given file and large enough to hold size
characters (when in doubt, use YY_BUF_SIZE
for the size). It returns a YY_BUFFER_STATE
handle, which may then be passed to other routines (see below). The
YY_BUFFER_STATE type is a pointer to an
opaque “struct yy_buffer_state” structure, so
YY_BUFFER_STATE variables may be safely
initialized to “((YY_BUFFER_STATE) 0)” if desired, and the
opaque structure can also be referred to in order to correctly declare input
buffers in source files other than that of scanners. Note that the
FILE pointer in the call to
yy_create_buffer() is only used as the
value of yyin seen by
YY_INPUT; if
YY_INPUT is redefined so that it no longer
uses yyin, then a nil
FILE pointer can safely be passed to
yy_create_buffer(). To select a particular
buffer to scan:

void yy_switch_to_buffer(YY_BUFFER_STATE
new_buffer)

It switches the scanner's input buffer so subsequent tokens will come from
new_buffer. Note that
yy_switch_to_buffer() may be used by
yywrap() to set things up for continued
scanning, instead of opening a new file and pointing
yyin at it. Note also that switching input
sources via either yy_switch_to_buffer() or
yywrap() does not change the start
condition.

void yy_delete_buffer(YY_BUFFER_STATE buffer)

is used to reclaim the storage associated with a buffer.
(buffer can be nil, in which case the routine
does nothing.) To clear the current contents of a buffer:

void yy_flush_buffer(YY_BUFFER_STATE buffer)

This function discards the buffer's contents, so the next time the scanner
attempts to match a token from the buffer, it will first fill the buffer anew
using YY_INPUT.

yy_new_buffer() is an alias for
yy_create_buffer(), provided for
compatibility with the C++ use of new and
delete for creating and destroying dynamic
objects.

Finally, the YY_CURRENT_BUFFER macro returns
a YY_BUFFER_STATE handle to the current
buffer.

Here is an example of using these features for writing a scanner which expands
include files (the ⟨⟨EOF⟩⟩ feature is discussed
below):

Three routines are available for setting up input buffers for scanning in-memory
strings instead of files. All of them create a new input buffer for scanning
the string, and return a corresponding
YY_BUFFER_STATE handle (which should be
deleted afterwards using
yy_delete_buffer()). They also switch to
the new buffer using yy_switch_to_buffer(),
so the next call to yylex() will start
scanning the string.

Note that both of these functions create and scan a copy of the string or bytes.
(This may be desirable, since yylex()
modifies the contents of the buffer it is scanning.) The copy can be avoided
by using:

yy_scan_buffer(char *base, yy_size_t size)

Which scans the buffer starting at base,
consisting of size bytes, the last two
bytes of which must be
YY_END_OF_BUFFER_CHAR (ASCII NUL).
These last two bytes are not scanned; thus, scanning consists of base[0]
through base[size-2], inclusive.

If base is not set up in this manner (i.e.,
forget the final two
YY_END_OF_BUFFER_CHAR bytes), then
yy_scan_buffer() returns a nil pointer
instead of creating a new input buffer.

The type yy_size_t is an integral type
which can be cast to an integer expression reflecting the size of the
buffer.

The special rule “⟨⟨EOF⟩⟩” indicates
actions which are to be taken when an end-of-file is encountered and
yywrap() returns non-zero (i.e., indicates
no further files to process). The action must finish by doing one of four
things:

Assigning yyin to a new input file (in
previous versions of flex, after doing
the assignment, it was necessary to call the special action
YY_NEW_FILE; this is no longer
necessary).

Executing a return statement.

Executing the special yyterminate()
action.

Switching to a new buffer using
yy_switch_to_buffer() as shown in the
example above.

⟨⟨EOF⟩⟩ rules may not be used with other patterns;
they may only be qualified with a list of start conditions. If an unqualified
⟨⟨EOF⟩⟩ rule is given, it applies to all start
conditions which do not already have ⟨⟨EOF⟩⟩
actions. To specify an ⟨⟨EOF⟩⟩ rule for only the
initial start condition, use

<INITIAL><<EOF>>

These rules are useful for catching things like unclosed comments. An example:

The macro YY_USER_ACTION can be defined to
provide an action which is always executed prior to the matched rule's action.
For example, it could be #define'd to call a routine to convert yytext to
lower-case. When YY_USER_ACTION is invoked,
the variable yy_act gives the number of the
matched rule (rules are numbered starting with 1). For example, to profile how
often each rule is matched, the following would do the trick:

#define YY_USER_ACTION
++ctr[yy_act]

where ctr is an array to hold the counts for
the different rules. Note that the macro
YY_NUM_RULES gives the total number of
rules (including the default rule, even if
-s is used), so a correct declaration for
ctr is:

int ctr[YY_NUM_RULES];

The macro YY_USER_INIT may be defined to
provide an action which is always executed before the first scan (and before
the scanner's internal initializations are done). For example, it could be
used to call a routine to read in a data table or open a logging file.

The macro yy_set_interactive(is_interactive)
can be used to control whether the current buffer is considered
interactive. An interactive buffer is processed
more slowly, but must be used when the scanner's input source is indeed
interactive to avoid problems due to waiting to fill buffers (see the
discussion of the -I flag below). A
non-zero value in the macro invocation marks the buffer as interactive, a zero
value as non-interactive. Note that use of this macro overrides
“%option always-interactive” or “%option
never-interactive” (see
OPTIONS below).
yy_set_interactive() must be invoked prior
to beginning to scan the buffer that is (or is not) to be considered
interactive.

The macro yy_set_bol(at_bol) can be used to
control whether the current buffer's scanning context for the next token match
is done as though at the beginning of a line. A non-zero macro argument makes
rules anchored with ‘^’ active, while a zero argument makes
‘^’ rules inactive.

The macro YY_AT_BOL returns true if the next
token scanned from the current buffer will have ‘^’ rules
active, false otherwise.

In the generated scanner, the actions are all gathered in one large switch
statement and separated using YY_BREAK,
which may be redefined. By default, it is simply a “break”, to
separate each rule's action from the following rules. Redefining
YY_BREAK allows, for example, C++ users to
“#define YY_BREAK” to do nothing (while being very careful that
every rule ends with a “break” or a “return”!) to
avoid suffering from unreachable statement warnings where because a rule's
action ends with “return”, the
YY_BREAK is inaccessible.

This section summarizes the various values available to the user in the rule
actions.

char *yytext

Holds the text of the current token. It may be modified but not lengthened
(characters cannot be appended to the end).

If the special directive “%array” appears in the first section
of the scanner description, then yytext
is instead declared “char yytext[YYLMAX]”, where
YYLMAX is a macro definition that can
be redefined in the first section to change the default value (generally
8KB). Using “%array” results in somewhat slower scanners,
but the value of yytext becomes immune to
calls to input() and
unput(), which potentially destroy its
value when yytext is a character pointer.
The opposite of “%array” is “%pointer”, which
is the default.

“%array” cannot be used when generating C++ scanner classes
(the -+ flag).

int yyleng

Holds the length of the current token.

FILE *yyin

Is the file which by default flex reads
from. It may be redefined, but doing so only makes sense before scanning
begins or after an EOF has been
encountered. Changing it in the midst of scanning will have unexpected
results since flex buffers its input;
use yyrestart() instead. Once scanning
terminates because an end-of-file has been seen,
yyin can be assigned as the new input
file and the scanner can be called again to continue scanning.

void yyrestart(FILE *new_file)

May be called to point yyin at the new
input file. The switch-over to the new file is immediate (any previously
buffered-up input is lost). Note that calling
yyrestart() with
yyin as an argument thus throws away the
current input buffer and continues scanning the same input file.

FILE *yyout

Is the file to which ECHO actions are done.
It can be reassigned by the user.

YY_CURRENT_BUFFER

Returns a YY_BUFFER_STATE handle to the
current buffer.

YY_START

Returns an integer value corresponding to the current start condition.
This value can subsequently be used with
BEGIN to return to that start condition.

One of the main uses of flex is as a
companion to the yacc(1)
parser-generator. yacc parsers expect to call a routine named
yylex() to find the next input token. The
routine is supposed to return the type of the next token as well as putting
any associated value in the global yylval,
which is defined externally, and can be a union or any other complex data
structure. To use flex with yacc, one
specifies the -d option to yacc to instruct
it to generate the file y.tab.h containing
definitions of all the “%tokens” appearing in the yacc input.
This file is then included in the flex
scanner. For example, if one of the tokens is “TOK_NUMBER”, part
of the scanner might look like:

Instructs flex to generate a 7-bit
scanner, i.e., one which can only recognize 7-bit characters in its input.
The advantage of using -7 is that the
scanner's tables can be up to half the size of those generated using the
-8 option (see below). The disadvantage
is that such scanners often hang or crash if their input contains an 8-bit
character.

Note, however, that unless generating a scanner using the
-Cf or
-CF table compression options, use of
-7 will save only a small amount of
table space, and make the scanner considerably less portable.
flex's default behavior is to generate
an 8-bit scanner unless -Cf or
-CF is specified, in which case
flex defaults to generating 7-bit
scanners unless it was configured to generate 8-bit scanners (as will
often be the case with non-USA sites). It is possible tell whether
flex generated a 7-bit or an 8-bit
scanner by inspecting the flag summary in the
-v output as described below.

Note that if -Cfe or
-CFe are used (the table compression
options, but also using equivalence classes as discussed below),
flex still defaults to generating an
8-bit scanner, since usually with these compression options full 8-bit
tables are not much more expensive than 7-bit tables.

Instructs flex to generate an 8-bit
scanner, i.e., one which can recognize 8-bit characters. This flag is only
needed for scanners generated using -Cf
or -CF, as otherwise
flex defaults to generating an 8-bit
scanner anyway.

See the discussion of -7 above for
flex's default behavior and the
tradeoffs between 7-bit and 8-bit scanners.

Instructs flex to generate a
batch scanner, the opposite of
interactive scanners generated by
-I (see below). In general,
-B is used when the scanner will never
be used interactively, and you want to squeeze a little more performance
out of it. If the aim is instead to squeeze out a lot more performance,
use the -Cf or
-CF options (discussed below), which
turn on -B automatically anyway.

Generate backing-up information to
lex.backup. This is a list of scanner
states which require backing up and the input characters on which they do
so. By adding rules one can remove backing-up states. If all backing-up
states are eliminated and -Cf or
-CF is used, the generated scanner will
run faster (see the -p flag). Only
users who wish to squeeze every last cycle out of their scanners need
worry about this option. (See the section on
PERFORMANCE
CONSIDERATIONS below.)

Instructs flex to trade off larger
tables in the generated scanner for faster performance because the
elements of the tables are better aligned for memory access and
computation. On some RISC architectures, fetching and manipulating
longwords is more efficient than with smaller-sized units such as
shortwords. This option can double the size of the tables used by the
scanner.

Directs flex to construct
equivalence classes, i.e., sets of
characters which have identical lexical properties (for example, if
the only appearance of digits in the
flex input is in the character
class “[0-9]” then the digits ‘0’,
‘1’, ‘...’, ‘9’ will all be
put in the same equivalence class). Equivalence classes usually give
dramatic reductions in the final table/object file sizes (typically a
factor of 2-5) and are pretty cheap performance-wise (one array
look-up per character scanned).

Directs flex to construct
meta-equivalence classes, which are sets
of equivalence classes (or characters, if equivalence classes are not
being used) that are commonly used together. Meta-equivalence classes
are often a big win when using compressed tables, but they have a
moderate performance impact (one or two “if” tests and
one array look-up per character scanned).

Causes the generated scanner to bypass
use of the standard I/O library (stdio) for input. Instead of calling
fread(3) or
getc(3), the scanner will
use the read(2) system
call, resulting in a performance gain which varies from system to
system, but in general is probably negligible unless
-Cf or
-CF are being used. Using
-Cr can cause strange behavior if,
for example, reading from yyin using
stdio prior to calling the scanner (because the scanner will miss
whatever text previous reads left in the stdio input buffer).

A lone -C specifies that the scanner
tables should be compressed but neither equivalence classes nor
meta-equivalence classes should be used.

The options -Cf or
-CF and
-Cm do not make sense together - there
is no opportunity for meta-equivalence classes if the table is not being
compressed. Otherwise the options may be freely mixed, and are cumulative.

The default setting is -Cem which
specifies that flex should generate
equivalence classes and meta-equivalence classes. This setting provides
the highest degree of table compression. It is possible to trade off
faster-executing scanners at the cost of larger tables with the following
generally being true:

Makes the generated scanner run in debug mode. Whenever a pattern is
recognized and the global yy_flex_debug
is non-zero (which is the default), the scanner will write to stderr a
line of the form:

--accepting rule at line 53 ("the matched
text")

The line number refers to the location of the rule in the file defining the
scanner (i.e., the file that was fed to
flex). Messages are also generated when
the scanner backs up, accepts the default rule, reaches the end of its
input buffer (or encounters a NUL; at this point, the two look the same as
far as the scanner's concerned), or reaches an end-of-file.

Specifies that the fast scanner table representation should be used (and
stdio bypassed). This representation is about as fast as the full table
representation (-f), and for some sets
of patterns will be considerably smaller (and for others, larger). In
general, if the pattern set contains both “keywords” and a
catch-all, “identifier” rule, such as in the set:

Instructs flex to generate an
interactive scanner. An interactive scanner
is one that only looks ahead to decide what token has been matched if it
absolutely must. It turns out that always looking one extra character
ahead, even if the scanner has already seen enough text to disambiguate
the current token, is a bit faster than only looking ahead when necessary.
But scanners that always look ahead give dreadful interactive performance;
for example, when a user types a newline, it is not recognized as a
newline token until they enter another token,
which often means typing in another whole line.

flex scanners default to
interactive unless
-Cf or
-CF table-compression options are
specified (see above). That's because if high-performance is most
important, one of these options should be used, so if they weren't,
flex assumes it is preferable to trade
off a bit of run-time performance for intuitive interactive behavior. Note
also that -I cannot be used in
conjunction with -Cf or
-CF. Thus, this option is not really
needed; it is on by default for all those cases in which it is allowed.

A scanner can be forced to not be interactive by using
-B (see above).

Instructs flex to generate a
case-insensitive scanner. The case of letters given in the
flex input patterns will be ignored,
and tokens in the input will be matched regardless of case. The matched
text given in yytext will have the
preserved case (i.e., it will not be folded).

Instructs flex not to generate
“#line” directives. Without this option,
flex peppers the generated scanner with
#line directives so error messages in the actions will be correctly
located with respect to either the original
flex input file (if the errors are due
to code in the input file), or lex.yy.c
(if the errors are flex's fault - these
sorts of errors should be reported to the email address given below).

Turns on maximum compatibility with the original AT&T
UNIXlex implementation. Note
that this does not mean full compatibility. Use of this option costs a
considerable amount of performance, and it cannot be used with the
-+,
-f,
-F,
-Cf, or
-CF options. For details on the
compatibilities it provides, see the section
INCOMPATIBILITIES
WITH LEX AND POSIX below. This option also results in the name
YY_FLEX_LEX_COMPAT being #define'd in
the generated scanner.

Directs flex to write the scanner to
the file output instead of
lex.yy.c. If
-o is combined with the
-t option, then the scanner is written
to stdout but its “#line” directives (see the
-L option above) refer to the file
output.

Changes the default “yy” prefix used by
flex for all globally visible variable
and function names to instead be prefix.
For example,
-Pfoo
changes the name of yytext to
footext. It also changes the name of the
default output file from lex.yy.c to
lex.foo.c. Here are all of the names
affected:

(If using a C++ scanner, then only yywrap
and yyFlexLexer are affected.) Within the
scanner itself, it is still possible to refer to the global variables and
functions using either version of their name; but externally, they have
the modified name.

This option allows multiple flex programs
to be easily linked together into the same executable. Note, though, that
using this option also renames
yywrap(), so now either an
(appropriately named) version of the routine for the scanner must be
supplied, or “%option noyywrap” must be used, as linking
with -lfl no longer provides one by
default.

Generates a performance report to stderr. The report consists of comments
regarding features of the flex input
file which will cause a serious loss of performance in the resulting
scanner. If the flag is specified twice, comments regarding features that
lead to minor performance losses will also be reported>

Note that the use of REJECT, “%option
yylineno”, and variable trailing context (see the
BUGS section below) entails a
substantial performance penalty; use of
yymore(), the ‘^’
operator, and the -I flag entail minor
performance penalties.

Causes the default rule (that unmatched scanner input is echoed to stdout)
to be suppressed. If the scanner encounters input that does not match any
of its rules, it aborts with an error. This option is useful for finding
holes in a scanner's rule set.

Makes flex run in
trace mode. It will generate a lot of
messages to stderr concerning the form of the input and the resultant
non-deterministic and deterministic finite automata. This option is mostly
for use in maintaining flex.

Specifies that flex should write to
stderr a summary of statistics regarding the scanner it generates. Most of
the statistics are meaningless to the casual
flex user, but the first line
identifies the version of flex (same as
reported by -V), and the next line the
flags used when generating the scanner, including those that are on by
default.

Specifies that flex should generate a
C++ scanner class. See the section on
GENERATING C++
SCANNERS below for details.

flex also provides a mechanism for
controlling options within the scanner specification itself, rather than from
the flex command line. This is done by
including “%option” directives in the first section of the
scanner specification. Multiple options can be specified with a single
“%option” directive, and multiple directives in the first
section of the flex input file.

Most options are given simply as names, optionally preceded by the word
“no” (with no intervening whitespace) to negate their meaning. A
number are equivalent to flex flags or
their negation:

Instructs flex to generate a scanner
which always considers its input “interactive”. Normally, on
each new input file the scanner calls
isatty() in an attempt to determine
whether the scanner's input source is interactive and thus should be read
a character at a time. When this option is used, however, no such call is
made.

main

Directs flex to provide a default
main() program for the scanner, which
simply calls yylex(). This option
implies “noyywrap” (see below).

never-interactive

Instructs flex to generate a scanner
which never considers its input “interactive” (again, no
call made to isatty()). This is the
opposite of “always-interactive”.

If set (i.e., “%option stdinit”), initializes
yyin and
yyout to stdin and stdout, instead of the
default of “nil”. Some existing
lex programs depend on this behavior,
even though it is not compliant with ANSI C, which does not require stdin
and stdout to be compile-time constant.

yylineno

Directs flex to generate a scanner that
maintains the number of the current line read from its input in the global
variable yylineno. This option is implied
by “%option lex-compat”.

yywrap

If unset (i.e., “%option noyywrap”), makes the scanner not
call yywrap() upon an end-of-file, but
simply assume that there are no more files to scan (until the user points
yyin at a new file and calls
yylex() again).

flex scans rule actions to determine whether
the REJECT or
yymore() features are being used. The
“reject” and “yymore” options are available to
override its decision as to whether to use the options, either by setting them
(e.g., “%option reject”) to indicate the feature is indeed used,
or unsetting them to indicate it actually is not used (e.g., “%option
noyymore”).

Three options take string-delimited values, offset with ‘=’:

%option outfile="ABC"

is equivalent to
-oABC, and

%option prefix="XYZ"

is equivalent to
-PXYZ.
Finally,

%option yyclass="foo"

only applies when generating a C++ scanner
(-+ option). It informs
flex that “foo” has been
derived as a subclass of yyFlexLexer, so
flex will place actions in the member
function “foo::yylex()” instead of
“yyFlexLexer::yylex()”. It also generates a
“yyFlexLexer::yylex()” member function that emits a run-time
error (by invoking “yyFlexLexer::LexerError()”) if called. See
GENERATING C++
SCANNERS, below, for additional information.

A number of options are available for lint purists who want to suppress the
appearance of unneeded routines in the generated scanner. Each of the
following, if unset (e.g., “%option nounput”), results in the
corresponding routine not appearing in the generated scanner:

The main design goal of flex is that it
generate high-performance scanners. It has been optimized for dealing well
with large sets of rules. Aside from the effects on scanner speed of the table
compression -C options outlined above,
there are a number of options/actions which degrade performance. These are,
from most expensive to least:

with the first three all being quite expensive and the last two being quite
cheap. Note also that unput() is
implemented as a routine call that potentially does quite a bit of work, while
yyless() is a quite-cheap macro; so if just
putting back some excess text, use
yyless().

REJECT should be avoided at all costs when
performance is important. It is a particularly expensive option.

Getting rid of backing up is messy and often may be an enormous amount of work
for a complicated scanner. In principal, one begins by using the
-b flag to generate a
lex.backup file. For example, on the input

The first few lines tell us that there's a scanner state in which it can make a
transition on an ‘o’ but not on any other character, and that in
that state the currently scanned text does not match any rule. The state
occurs when trying to match the rules found at lines 2 and 3 in the input
file. If the scanner is in that state and then reads something other than an
‘o’, it will have to back up to find a rule which is matched.
With a bit of headscratching one can see that this must be the state it's in
when it has seen ‘fo’. When this has happened, if anything other
than another ‘o’ is seen, the scanner will have to back up to
simply match the ‘f’ (by the default rule).

The comment regarding State #8 indicates there's a problem when
“foob” has been scanned. Indeed, on any character other than an
‘a’, the scanner will have to back up to accept
“foo”. Similarly, the comment for State #9 concerns when
“fooba” has been scanned and an ‘r’ does not
follow.

The final comment reminds us that there's no point going to all the trouble of
removing backing up from the rules unless we're using
-Cf or
-CF, since there's no performance gain
doing so with compressed scanners.

Backing up messages tend to cascade. With a complicated set of rules it's not
uncommon to get hundreds of messages. If one can decipher them, though, it
often only takes a dozen or so rules to eliminate the backing up (though it's
easy to make a mistake and have an error rule accidentally match a valid
token; a possible future flex feature will
be to automatically add rules to eliminate backing up).

It's important to keep in mind that the benefits of eliminating backing up are
gained only if every instance of backing up is
eliminated. Leaving just one gains nothing.

Variable trailing context (where both the leading
and trailing parts do not have a fixed length) entails almost the same
performance loss as REJECT (i.e., substantial).
So when possible a rule like:

%%
mouse|rat/(cat|dog) run();

is better written:

%%
mouse/cat|dog run();
rat/cat|dog run();

or as

%%
mouse|rat/cat run();
mouse|rat/dog run();

Note that here the special ‘|’ action does not provide any
savings, and can even make things worse (see
BUGS below).

Another area where the user can increase a scanner's performance (and one that's
easier to implement) arises from the fact that the longer the tokens matched,
the faster the scanner will run. This is because with long tokens the
processing of most input characters takes place in the (short) inner scanning
loop, and does not often have to go through the additional work of setting up
the scanning environment (e.g., yytext) for
the action. Recall the scanner for C comments:

Now instead of each newline requiring the processing of another action,
recognizing the newlines is “distributed” over the other rules
to keep the matched text as long as possible. Note that adding rules does
not slow down the scanner! The speed of the
scanner is independent of the number of rules or (modulo the considerations
given at the beginning of this section) how complicated the rules are with
regard to operators such as ‘*’ and ‘|’.

A final example in speeding up a scanner: scan through a file containing
identifiers and keywords, one per line and with no other extraneous
characters, and recognize all the keywords. A natural first approach is:

One has to be careful here, as we have now reintroduced backing up into the
scanner. In particular, while we know that there will never be any characters
in the input stream other than letters or newlines,
flex can't figure this out, and it will
plan for possibly needing to back up when it has scanned a token like
“auto” and then the next character is something other than a
newline or a letter. Previously it would then just match the
“auto” rule and be done, but now it has no “auto”
rule, only an “auto\n” rule. To eliminate the possibility of
backing up, we could either duplicate all rules but without final newlines or,
since we never expect to encounter such an input and therefore don't how it's
classified, we can introduce one more catch-all rule, this one which doesn't
include a newline:

Compiled with -Cf, this is about as fast as
one can get a flex scanner to go for this
particular problem.

A final note: flex is slow when matching
NUL's, particularly when a token contains multiple NUL's. It's best to write
rules which match short amounts of text if it's anticipated that the text will
often include NUL's.

Another final note regarding performance: as mentioned above in the section
HOW THE INPUT IS
MATCHED, dynamically resizing yytext to
accommodate huge tokens is a slow process because it presently requires that
the (huge) token be rescanned from the beginning. Thus if performance is
vital, it is better to attempt to match “large” quantities of
text but not “huge” quantities, where the cutoff between the two
is at about 8K characters/token.

flex provides two different ways to generate
scanners for use with C++. The first way is to simply compile a scanner
generated by flex using a C++ compiler
instead of a C compiler. This should not generate any compilation errors
(please report any found to the email address given in the
AUTHORS section below). C++ code
can then be used in rule actions instead of C code. Note that the default
input source for scanners remains yyin, and
default echoing is still done to yyout. Both
of these remain FILE * variables and not C++
streams.

flex can also be used to generate a C++
scanner class, using the -+ option (or,
equivalently, “%option c++”), which is automatically specified
if the name of the flex executable ends in a ‘+’, such as
flex++. When using this option,
flex defaults to generating the scanner to
the file lex.yy.cc instead of
lex.yy.c. The generated scanner includes
the header file
<g++/FlexLexer.h>,
which defines the interface to two C++ classes.

The first class, FlexLexer, provides an abstract
base class defining the general scanner class interface. It provides the
following member functions:

const char* YYText()

Returns the text of the most recently matched token, the equivalent of
yytext.

int YYLeng()

Returns the length of the most recently matched token, the equivalent of
yyleng.

int lineno() const

Returns the current input line number (see “%option
yylineno”), or 1 if “%option yylineno” was not
used.

void set_debug(int flag)

Sets the debugging flag for the scanner, equivalent to assigning to
yy_flex_debug (see the
OPTIONS section above). Note
that the scanner must be built using “%option debug” to
include debugging information in it.

int debug() const

Returns the current setting of the debugging flag.

Also provided are member functions equivalent to
yy_switch_to_buffer(),
yy_create_buffer() (though the first
argument is an std::istream* object pointer
and not a FILE*),
yy_flush_buffer(),
yy_delete_buffer(), and
yyrestart() (again, the first argument is
an std::istream* object pointer).

The second class defined in
<g++/FlexLexer.h>
is yyFlexLexer, which is derived from
FlexLexer. It defines the following
additional member functions:

yyFlexLexer(std::istream* arg_yyin = 0, std::ostream* arg_yyout = 0)

Constructs a yyFlexLexer object using the
given streams for input and output. If not specified, the streams default
to cin and
cout, respectively.

virtual int yylex()

Performs the same role as yylex() does
for ordinary flex scanners: it scans the input stream, consuming tokens,
until a rule's action returns a value. If subclass ‘S’ is
derived from yyFlexLexer, in order to
access the member functions and variables of ‘S’ inside
yylex(), use “%option
yyclass="S"” to inform
flex that the ‘S’
subclass will be used instead of
yyFlexLexer. In this case, rather than
generating “yyFlexLexer::yylex()”,
flex generates
“S::yylex()” (and also generates a dummy
“yyFlexLexer::yylex()” that calls
“yyFlexLexer::LexerError()” if called).

Reassigns yyin to
new_in (if non-nil) and
yyout to
new_out (ditto), deleting the previous
input buffer if yyin is reassigned.

int yylex(std::istream* new_in, std::ostream* new_out = 0)

First switches the input streams via “switch_streams(new_in,
new_out)” and then returns the value of
yylex().

In addition, yyFlexLexer defines the following
protected virtual functions which can be redefined in derived classes to
tailor the scanner:

virtual int LexerInput(char* buf, int max_size)

Reads up to max_size characters into
buf and returns the number of characters
read. To indicate end-of-input, return 0 characters. Note that
“interactive” scanners (see the
-B and
-I flags) define the macro
YY_INTERACTIVE. If
LexerInput() has been redefined, and
it's necessary to take different actions depending on whether or not the
scanner might be scanning an interactive input source, it's possible to
test for the presence of this name via “#ifdef”.

virtual void LexerOutput(const char* buf, int size)

Writes out size characters from the
buffer buf, which, while NUL-terminated,
may also contain “internal” NUL's if the scanner's rules can
match text with NUL's in them.

virtual void LexerError(const char* msg)

Reports a fatal error message. The default version of this function writes
the message to the stream cerr and
exits.

Note that a yyFlexLexer object contains its
entire scanning state. Thus such objects can be used to create reentrant
scanners. Multiple instances of the same
yyFlexLexer class can be instantiated, and
multiple C++ scanner classes can be combined in the same program using the
-P option discussed above.

Finally, note that the “%array” feature is not available to C++
scanner classes; “%pointer” must be used (the default).

To create multiple (different) lexer classes, use the
-P flag (or the “prefix=”
option) to rename each yyFlexLexer to some
other xxFlexLexer.
<g++/FlexLexer.h>
can then be included in other sources once per lexer class, first renaming
yyFlexLexer as follows:

flex is a rewrite of the
AT&T UNIXlex
tool (the two implementations do not share any code, though), with some
extensions and incompatibilities, both of which are of concern to those who
wish to write scanners acceptable to either implementation.
flex is fully compliant with the POSIX
lex specification, except that when using
“%pointer” (the default), a call to
unput() destroys the contents of
yytext, which is counter to the POSIX
specification.

In this section we discuss all of the known areas of incompatibility between
flex, AT&T UNIXlex, and the POSIX specification.

flex's -l
option turns on maximum compatibility with the original
AT&T UNIXlex
implementation, at the cost of a major loss in the generated scanner's
performance. We note below which incompatibilities can be overcome using the
-l option.

yylineno should be maintained on a
per-buffer basis, rather than a per-scanner (single global variable)
basis.

yylineno is not part of the POSIX
specification.

The input() routine is not redefinable,
though it may be called to read characters following whatever has been
matched by a rule. If input()
encounters an end-of-file, the normal
yywrap() processing is done. A
“real” end-of-file is returned by
input() as
EOF.

Input is instead controlled by defining the
YY_INPUT macro.

The flex restriction that
input() cannot be redefined is in
accordance with the POSIX specification, which simply does not specify any
way of controlling the scanner's input other than by making an initial
assignment to yyin.

The unput() routine is not redefinable.
This restriction is in accordance with POSIX.

flex scanners are not as reentrant as
lex scanners. In particular, if a
scanner is interactive and an interrupt handler long-jumps out of the
scanner, and the scanner is subsequently called again, the following error
message may be displayed:

fatal flex scanner internal error--end of buffer
missed

To reenter the scanner, first use

yyrestart(yyin);

Note that this call will throw away any buffered input; usually this isn't a
problem with an interactive scanner.

Also note that flex C++ scanner classes are reentrant, so if using C++ is an
option , they should be used instead. See
GENERATING C++
SCANNERS above for details.

output() is not supported. Output from
the ECHO macro is done to the file-pointer
yyout (default stdout).

output() is not part of the POSIX
specification.

lex does not support exclusive start
conditions (%x), though they are in the POSIX specification.

When definitions are expanded, flex
encloses them in parentheses. With lex,
the following:

NAME [A-Z][A-Z0-9]*
%%
foo{NAME}? printf("Found it\n");
%%

will not match the string “foo” because when the macro is
expanded the rule is equivalent to “foo[A-Z][A-Z0-9]*?” and
the precedence is such that the ‘?’ is associated with
“[A-Z0-9]*”. With flex,
the rule will be expanded to “foo([A-Z][A-Z0-9]*)?” and so
the string “foo” will match.

Note that if the definition begins with ‘^’ or ends with
‘$’ then it is not expanded with parentheses, to allow these
operators to appear in definitions without losing their special meanings.
But the ‘⟨s⟩’, ‘/’, and
⟨⟨EOF⟩⟩ operators cannot be used in a
flex definition.

Using -l results in the
lex behavior of no parentheses around
the definition.

The POSIX specification is that the definition be enclosed in
parentheses.

Some implementations of lex allow a
rule's action to begin on a separate line, if the rule's pattern has
trailing whitespace:

%%
foo|bar<space here>
{ foobar_action(); }

flex does not support this feature.

The lex ‘%r’ (generate a
Ratfor scanner) option is not supported. It is not part of the POSIX
specification.

After a call to unput(),
yytext is undefined until the next token
is matched, unless the scanner was built using “%array”.
This is not the case with lex or the
POSIX specification. The -l option does
away with this incompatibility.

The precedence of the ‘{}’ (numeric range) operator is
different. lex interprets
“abc{1,3}” as match one, two, or three occurrences of
‘abc’, whereas flex
interprets it as match ‘ab’ followed by one, two, or three
occurrences of ‘c’. The latter is in agreement with the
POSIX specification.

The precedence of the ‘^’ operator is different.
lex interprets “^foo|bar”
as match either ‘foo’ at the beginning of a line, or
‘bar’ anywhere, whereas
flex interprets it as match either
‘foo’ or ‘bar’ if they come at the beginning
of a line. The latter is in agreement with the POSIX specification.

The special table-size declarations such as ‘%a’ supported
by lex are not required by
flex scanners;
flex ignores them.

The name FLEX_SCANNER is #define'd so
scanners may be written for use with either
flex or
lex. Scanners also include
YY_FLEX_MAJOR_VERSION and
YY_FLEX_MINOR_VERSION indicating which
version of flex generated the scanner
(for example, for the 2.5 release, these defines would be 2 and 5,
respectively).

The following flex features are not included
in lex or the POSIX specification:

plus almost all of the flex flags. The last
feature in the list refers to the fact that with
flex multiple actions can be placed on the
same line, separated with semi-colons, while with
lex, the following

foo handle_foo();
++num_foos_seen;

is (rather surprisingly) truncated to

foo handle_foo();

flex does not truncate the action. Actions
that are not enclosed in braces are simply terminated at the end of the line.

Indicates that the given rule cannot be matched because it follows other
rules that will always match the same text as it. For example, in the
following “foo” cannot be matched because it comes after an
identifier “catch-all” rule:

[a-z]+ got_identifier();
foo got_foo();

Using REJECT in a scanner suppresses this
warning.

warning, -s option given but default rule can be matched

Means that it is possible (perhaps only in a particular start condition)
that the default rule (match any single character) is the only one that
will match a particular input. Since -s
was given, presumably this is not intended.

reject_used_but_not_detected undefined

yymore_used_but_not_detected undefined

These errors can occur at compile time. They indicate that the scanner
uses REJECT or
yymore() but that
flex failed to notice the fact, meaning
that flex scanned the first two
sections looking for occurrences of these actions and failed to find any,
but somehow they snuck in (via an #include file, for example). Use
“%option reject” or “%option yymore” to
indicate to flex that these features
are really needed.

flex scanner jammed

A scanner compiled with -s has
encountered an input string which wasn't matched by any of its rules. This
error can also occur due to internal problems.

token too large, exceeds YYLMAX

The scanner uses “%array” and one of its rules matched a
string longer than the YYLMAX constant
(8K bytes by default). The value can be increased by #define'ing
YYLMAX in the definitions section of
flex input.

scanner requires -8 flag to use the character 'x'

The scanner specification includes recognizing the 8-bit character
‘x’ and the -8 flag was
not specified, and defaulted to 7-bit because the
-Cf or
-CF table compression options were
used. See the discussion of the -7 flag
for details.

flex scanner push-back overflow

unput() was used to push back so much text that the scanner's buffer could
not hold both the pushed-back text and the current token in
yytext. Ideally the scanner should
dynamically resize the buffer in this case, but at present it does
not.

Vern Paxson, with the help of many ideas and much inspiration from Van Jacobson.
Original version by Jef Poskanzer. The fast table representation is a partial
implementation of a design done by Van Jacobson. The implementation was done
by Kevin Gong and Vern Paxson.

Thanks to Esmond Pitt and Earle Horton for 8-bit character support; to Benson
Margulies and Fred Burke for C++ support; to Kent Williams and Tom Epperly for
C++ class support; to Ove Ewerlid for support of NUL's; and to Eric Hughes for
support of multiple buffers.

This work was primarily done when I was with the Real Time Systems Group at the
Lawrence Berkeley Laboratory in Berkeley, CA. Many thanks to all there for the
support I received.

Some trailing context patterns cannot be properly matched and generate warning
messages (dangerous trailing context). These are patterns where the ending of
the first part of the rule matches the beginning of the second part, such as
“zx*/xy*”, where the ‘x*’ matches the
‘x’ at the beginning of the trailing context. (Note that the
POSIX draft states that the text matched by such patterns is undefined.)

For some trailing context rules, parts which are actually fixed-length are not
recognized as such, leading to the above mentioned performance loss. In
particular, parts using ‘|’ or ‘{n}’ (such as
“foo{3}”) are always considered variable-length.

Combining trailing context with the special ‘|’ action can result
in fixed trailing context being turned into the more expensive variable
trailing context. For example, in the following:

%%
abc |
xyz/def

Use of unput() invalidates yytext and yyleng,
unless the “%array” directive or the
-l option has been used.

Pattern-matching of NUL's is substantially slower than matching other
characters.

Dynamic resizing of the input buffer is slow, as it entails rescanning all the
text matched so far by the current (generally huge) token.

Due to both buffering of input and read-ahead, it is not possible to intermix
calls to
<stdio.h>
routines, such as, for example, getchar(),
with flex rules and expect it to work. Call
input() instead.

The total table entries listed by the -v flag
excludes the number of table entries needed to determine what rule has been
matched. The number of entries is equal to the number of DFA states if the
scanner does not use REJECT, and somewhat greater
than the number of states if it does.